The annealing and spheroidizing of rod and wire in coil form is accomplished using one of two basic furnace design concepts. The first type of design is called "batch" heat-treatment, and as the name implies, the product is thermally treated as a unit or batch. The second design is termed "continuous type furnaces", and these are usually known for higher productivity and improved uniformity within the heat-treated product.
The invention to be disclosed herein falls into the second category. The continuous annealing furnaces are typically fired by natural gas heating radiant tubes, with a protective atmosphere around the product, and pusher trays which carry the product through the furnace on tracks.
Thermal treatment is a function of time and temperature. Once a treatment has been established for a given product, the treatment time is relatively fixed. The capacity of a furnace is determined by the charge weight and the cycle rate. Usually, both the charge weight and the cycle rate are fixed.
There are two basic types of thermal treatment: annealing and spheroidizing. Annealing is characterized by faster cycle rates and lower product ductility, while spheroidization is a slower process and produces higher product ductility.
Furnace capacity is therefore the result of the mix of annealing a single spheroidizing cycles. As the volume of spheroidization increases, the furnace capacity decreases.
Some capacity improvement has been obtained by the manipulation of the temperatures within the process, allowing for a shorter cycle time. Once optimum time and temperature relationships have been established, rates can be considered as fixed.
The present invention was developed in relation to an existing furnace through which product is transported on wheeled trays travelling on two sets of parallel tracks. Each tray is 56 inches square and consists of articulated sections of heat-resistant stainless steel. The furnace chamber holds 34 trays in a configuration of side-by-side pairs, giving 17 tray positions longitudinally of the furnace.
The rationale behind the development of the present invention can be summarized as follows. Firstly, it must be understood that the trays described in the previous paragraph are sized to accommodate the maximum outside diameter of the material to be processed. However, many continuous annealing furnaces are used to process material having a smaller outside diameter than the maximum size. When the smaller outside diameter material is being processed, the result is the waste of a portion of the space on the trays. Generally speaking, there is not enough room to put two coils on a tray. The inventive leap is that, with small O.D. material, it would be possible to provide three coils on two trays, with one of the coils straddling the trays. For this to be done, the tray movement must be absolutely synchronized, to avoid differential forward motion and the risk that the straddling coil could be "walked" off the trays, or at least out of its ideal position. Thus it became necessary to engineer the synchronized movement of the trays, as well as to engineer modifications to the furnace loading and unloading equipment.
It was expected that, by taking the foregoing steps, the capacity of the furnace could be increased by about 30% to 50% depending on cycle and product mix. Because a typical furnace of the kind under discussion utilizes separately operated hydraulic cylinders to push the trays through the furnace along the side-by-side parallel tracks, it was necessary to provide some means to avoid the tray misalignment that would result.